The mechanisms regulating the onset of follicular morphogenesis remain unresolved, but the differentiation of granulosa cells from pluripotential somatic cells and their interaction with oocytes appears to be an essential early step. In contrast to rats and mice, ovaries of newborn Syrian hamsters contain oocytes in the 1st meiotic prophase, and undifferentiated somatic cells. Therefore, we have an ideal animal model for studying the mechanism involved in producing the first cohort of follicles. Preliminary studies have shown that: [1] immunoinactivation of endogenous FSH results in nearly complete inhibition of primordial follicle formation and this can be reversed by equine chorionic gonadotropin (eCG), a hormone with FSH activity unaffected by the highly specific anti-FSH serum; [2] the failure of follicular formation is correlated with an appreciable reduction in the transcription and translation of the epidermal growth factor (EGF) receptor gene consistent with our earlier findings that FSH functions via EGF and its receptor; [3] exposure of neonatal ovaries in vivo or in vitro to eCG upregulates the expression of EGF, EGF-receptor and TGF-beta receptor associated with accelerated granulosa cell and follicle differentiation and [4] increased expression of FSH-receptor mRNA correlates with increased FSH function. We hypothesize that FSH controls the differentiation of pluripotential somatic cells into granulosa cells during the first phases of follicular morphogenesis by mechanisms that involve the spatio-temporal interaction of EGF and TGF-beta receptors with their appropriate ligands. This will be tested in vivo using anti-FSH antiserum and in vitro using fetal and early postnatal hamster ovaries exposed to FSH and/or growth factors. Measured parameters include [1] morphometric quantitation of follicle formation; [2] quantitative evaluation by Western immunoblotting, of the significance of EGF and TGF-beta ligand and receptor expression during granulosa cell differentiation; their cell- specific subtle expression will be identified by immunofluorescence; [3] correlation of changes in EGF and TGF-beta receptor mRNA, determined by RT-PCR quantitation, with gonadotropin-regulated differentiation and [4] measurement of ovarian steroidogenic activity to determine the functional significance of follicular development following in vitro hormone and/or growth factor manipulation. Results will contribute substantially to our understanding of the mechanisms involved in the initial steps of folliculogenesis, i.e., the formation of primordial follicles, which will prove valuable for improving the treatment and management of human infertility.